The manufacture of printed circuit boards requires a number of steps to form the desired end product since the board is, in effect, a multilayer configuration of conductor lines, resist layers, solder masks, etc. Regardless of the method of manufacture, however, liquid coatings requiring drying before further processing must be applied to the board surface at various points in the process.
One particular coating used in the fabrication of high quality printed circuit boards is a solder mask which serves to protect the circuitry from bridging together during soldering and which must be resistant to solvents, chemicals and flux compositions. For convenience, the following description will be directed to this particular material and its application to the board by conventional screen printing techniques although it will be appreciated by those skilled in the art that other liquids and other methods of application could also be employed.
Basically, the screen printing technique involves squeezing a composition through the open meshes of a stretched piece of material such as wire onto a printable substrate. The screen is covered or blocked out in part by a masking material in order to form the desired pattern on the printable substrate. The masking material may simply be a stencil or a dried lacquer, shellac or glue. Once the screen has been covered or blocked out in part by a masking material, it is held taut on a frame and positioned over the desired substrate. The coating composition is then poured onto the screen and squeezed through the open areas with a squeegee. Thereafter, the frame is removed and the coated substrate baked. This procedure is then repeated for the other side of the board.
It is important, when following a desired pattern, that the composition does not flow or bleed outside of the preselected areas defined by the open area of the screen but should follow accurately the image formed on the screen and reproduce it. These known techniques to produce screen coatings are described for example in U.S. Pat. No. 4,292,230, which patent is hereby incorporated by reference.
As noted above, and as specified by manufacturers of the screen printing compositions, e.g., photo-imageable solder masks, the material is applied to one side of a printed circuit board, baked (dried) in an oven, and the process repeated for the other side of the board. Both sides cannot now be done simultaneously because the screen printed board is wet and must be dried before it can be inverted and coated on the opposite side.
Aside from the inefficiencies with having to perform two drying cycles and associated processing stages, one side is necessarily subjected to the two drying cycles and unless care is exercised to control the drying temperature and duration, the subsequent development cycle to remove the solder mask from areas designated by the printed circuit board artwork may not be successful and a residue may remain on the surfaces that are required to be clean copper.
A number of patents have issued addressing the problem of coating the surfaces of boards but these require complicated machines and complicated procedures to perform the process. For example, U.S. Pat. No. 4,600,601 uses an apparatus which passes an inclined board through a shower of coating liquid and then pivots the board and feeds the other side of the inclined board through the shower. U.S. Pat. No. 4,678,531 shows a method for screen printing an electrical fabrication substance onto a substrate surface to which a device having a substantial height relative to the thickness of the printing screen had been previously mounted. U.S. Pat. No. 4,436,806 is directed to making a resist pattern on circuit boards using a photo tool in registration with the circuit board blanks while the image is still wet. The uncured polymer which remains liquid is removed after imaging so that the board can be processed by etch resist, plate resist or solder mask techniques. U.S. Pat. No. 4,031,268 shows a method of spraying metallic patterns onto a substrate using a template.
The need still remains, however, for a method for preparing printed circuit boards in an efficient and economical manner.
It is therefore an object of the present invention to provide a method for applying liquid coatings sequentially on both surfaces of printed circuit boards, which coatings may then be dried in one drying step.
It is another object of this invention to provide an article of manufacture which may be used to apply liquid coatings on printed circuit boards.
These and other objects and advantages of the present invention will become readily apparent from the following description and illustrative embodiments.